The anticancer prodrug CPT-11 is activated by esterases to yield the potent topoisomerase I inhibitor SN-3S. Expression of a rabbit liver carboxylesterase (rCE) that can efficiently activate CPT-11 confers sensitivity to the drug both to cells in culture and when grown as xenografts in immune-deprived mice. Since the activation of this agent in humans is poor, this affords the potential to use rCE in combination with CPT- 11 in a virus directed enzyme prodrug therapy approach (VDEPT). This proposal expands on previous studies to determine the effectiveness of VDEPT in eliminating human tumor cells from immune-deprived animals in a selective fashion. The premise for these studies is that tumor cells frequently overexpress the oncogene c-MYC, a transcription factor. By designing appropriate expression vectors based upon the omithine decarboxylase promoter, a gene specifically upregulated by c-myc, we propose to achieve tumor selective expression of the rCE (or hiCE) and hence selectively sensitize these cells to CPT-11. The experiments in this application propose to identify the most efficient carboxylesterase and transcriptional control elements to achieve tumor selective expression and to design adenovirus as delivery vehicles. The Specific Aims of this application are, therefore: 1) To compare the abilities of the previously described rabbit CE (rCE) and a recently isolated human intestinal CE (hi CE) to sensitize human tumor cells to CPT- 11; 2) To specifically sensitize human tumor cells that express c-MYC to CPT-11 by expression of rCE orhiCE under the control of a modified omithine ecarboxylase promoter; and 3) To eliminate minimal residual disease and bulk tumor in a human tumor xenograft animal model system using adenovirus demonstrating specific expression of rCE or hiCE in combination with CPT-11. These studies will assess the effectiveness of VDEPT with rCE or hiCE and CPT-11 at eliminating tumor cells from appropriate animal models of human disease.